Lightning is a sudden electrostatic discharge that occurs during a thunderstorm. This discharge occurs between electrically charged regions of a cloud (called intra-cloud lightning or IC), between two clouds (CC lightning), or between a cloud and the ground (CG lightning). The charged regions in the atmosphere temporarily equalize themselves through this discharge, which is referred to as a strike if it hits an object on the ground, and referred to as a flash, if it occurs within a cloud.
The main charging area in a thunderstorm occurs in the central part of the storm where air is moving upward rapidly (updraft) and temperatures range from −15 to −25 Celsius. At that central part, the combination of temperature and rapid upward air movement produces a mixture of super-cooled cloud droplets (small water droplets below freezing), small ice crystals, and soft hail (graupel). The updraft carries the super-cooled cloud droplets and very small ice crystals upward. At the same time, the graupel, which is considerably larger and denser, tends to fall or be suspended in the rising air. The differences in the movement of the precipitation cause collisions to occur. When the rising ice crystals collide with graupel (soft hail), the ice crystals become positively charged and the graupel becomes negatively charged.
The updraft carries the positively charged ice crystals upward toward the top of the storm cloud. The larger and denser graupel is either suspended in the middle of the thunderstorm cloud or falls toward the lower part of the storm. As a result, the upper part of the thunderstorm cloud becomes positively charged while the middle to lower part of the thunderstorm cloud becomes negatively charged.
As a thunderstorm cloud moves over the surface of the Earth, an equal electric charge, but of opposite polarity, is induced on the Earth's surface underneath the cloud. The oppositely charged regions create an electric field within the air between them. This electric field varies in relation to the strength of the surface charge on the base of the thundercloud, such that the greater the accumulated charge, the higher the electrical field.
Under some conditions, a channel of ionized air, called a “leader”, is initiated from a charged region in the thunderstorm cloud. Leaders are electrically conductive channels of partially ionized gas that travel away from a region of dense charge. Negative leaders propagate away from densely charged regions of negative charge, and positive leaders propagate from positively charged regions. The positively and negatively charged leaders proceed in opposite directions, a positive leader proceeds upwards within the cloud and a negative leader proceeds towards the earth.
When a leader approaches the ground, the presence of opposite charges at points on the ground enhances the strength of the electric field. If the electric field is strong enough, a positively charged ionic channel, called a positive upward leader (also referred to as an upward streamer), can develop from these points.
Once a downward leader connects to an available upward leader, a process referred to as attachment, a low resistance path is formed and discharge may occur. Once a conductive channel bridges the air gap between the negative charge excess in the cloud and the positive surface charge excess below, an electrical discharge follows. This electric discharge may be referred to as a “strike” and it is the luminous and noticeable part of the lightning discharge.
An aircraft flying through a thunderstorm may be struck by a lightning while in flight. There are two kinds of lightning encounters by an aircraft. The first one may be referred to as an intercepted lightning, which occurs when the flight path of the aircraft intercepts a lightning strike that is already in progress. The second and more common type is an aircraft-induced lightning, where the aircraft disrupts the electric field in the atmosphere and initiate an electric spark that may cause a lightning strike on the aircraft. Regardless of the type of strike, the lightning may strike the aircraft anywhere and may affect operation of the aircraft.
In an example, if an aircraft is made of a metal (e.g., aluminum), the skin or outer structure of the aircraft may be made thick so as to enable the aircraft to withstand the strike. In another example, if the skin or outer structure of the aircraft is made of a composite material, the composite material may be embedded with a mesh of a metallic conductor (e.g., a copper mesh). The entire structure may be embedded with such a mesh because the location of a strike could be anywhere on the aircraft.
Making the body of the aircraft from a thick metal or embedding a composite structure with a copper mesh to protect the aircraft from lightning increases the weight of the aircraft. Increasing the weight of the aircraft reduces the fuel efficiency of the aircraft. Therefore, it may be desirable to include a lightning strike protection system in the aircraft that predictably causes any lightning strike to hit the aircraft at a known location and facilitates guiding the strike safely across the aircraft and discharging the strike away from the aircraft.